This application claims the priority of German application 198 41 089.1, filed in Germany on Sep. 9, 1998, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to an arrangement for condensing a drafted fiber strand in a condensing zone arranged downstream of a front roller pair of a drafting unit, which condensing zone comprises a stationary sliding surface having a suction slit extending essentially in transport direction of the fiber strand, a perforated transport belt which transports the fiber strand over the sliding surface, and a nipping roller engageable with the transport belt on an exit side of the condensing zone.
An arrangement of this type is prior art in U.S. Pat. No. 5,600,872. The condensing zone serves the purpose of further bundling and condensing the drafted fiber strand, whereby the outwardly projecting fiber ends are rolled in around the core strand. The fiber strand is still untwisted in the condensing zone. When the spinning twist is later applied downstream of the nipping roller, a thread arises which is less hairy and more tear resistant and more even. In the known arrangement, the nipping roller is pressed against a driven bottom cylinder. As a result, the suction slit ends a significant distance from the nipping point of the nipping roller. The condensed fiber strand can thus, disadvantageously, expand outwards again before it reaches the nipping point. Thus the actual purpose of the condensing zone is only partly achieved.
It is an object of the present invention to design the condensing zone in such a way that the condensed fiber strand maintains its state up until it reaches the nipping point. It is also an object of the present invention that in the case of a it possible break in the transport belt, the nipping roller is not damaged.
This object has been achieved in accordance with the present invention in that the nipping roller presses the transport belt to the sliding surface, said nipping roller being connected to a drive from which it can be cut off in the case of a break in the transport belt.
Because the nipping roller does not act together with another delivery roller, but rather with a stationary sliding surface, the suction slit can be placed right up to the nipping point. This brings the advantage with it that the pneumatically generated condensing of the fiber strand is maintained at the nipping point also, so that subsequently, a truly condensed fiber strand can be twisted to form a thread. In order that, in the case of break in the transport belt, the nipping roller, as a rule provided with a flexible coating, does not rub on the stationary sliding surface and by means of overheating is thus destroyed, it is provided that the nipping roller drive is stopped once a break in the transport belt occurs.
To this end, preferred embodiments of the drive comprises a transfer wheel which can be de-coupled by means of axial displacement from its coupled position with a driven wheel of the nipping roller. This can be achieved, for example, in that the transfer wheel as well as the driven wheel are both provided with a helical gearing. In the case of a helical gearing, an axial force arises which is proportional to the transferable torque. A spring element effects that the axial force during normal operation does not exceed the spring force. In contrast, in the case of a break in the transport belt, the axial force of the helical gearing increases due to the increased friction, so that the transfer wheel is axially displaced due to the greater torque, whereby the engagement between the transfer wheel and the driven wheel is interrupted. The spring force of the spring element must be of such a magnitude that it is greater than the axial force during normal operation, but less than the axial force in the case of a break in the transport belt.
Advantageously, a positioning device for establishing the de-coupled position is arranged at the spring element while the transfer wheel is in a de-coupled state. This ensures that the spring force does not press the transfer wheel back into the operational position again as long as the transport belt is not in perfect working order. It is advantageous when the positioning device comprises a holding magnet according to certain preferred embodiments of the invention. This can on the one hand reinforce the axial force at the end of the displacement motion, while on the other hand, it is in the position to hold the transfer wheel in the de-coupled position. The magnetic force must hereby be greater than the spring force effective during operation.